1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus for transferring an image onto a print medium.
2. Description of Related Art
Conventional image forming apparatuses form toner images generally by a method as described below. First, an optical scanning device emits a beam to an electrified photosensitive drum, whereby an electrostatic latent image is formed on the surface of the photosensitive drum. Next, a developing device supplies toner to the surface of the photosensitive drum, whereby the electrostatic latent image is developed into a toner image. Meanwhile, a feed roller picks up a sheet from a stack of sheets stored in a sheet tray. Next, a pair of registration rollers feeds the sheet picked up by the feed roller. A transfer roller, which is disposed opposite to the photosensitive drum, transfers the toner image from the surface of the photosensitive drum to the sheet fed by the pair of registration rollers. Next, the sheet is subjected to a heat/pressure treatment performed by a fixing device, whereby the toner image is fixed on the sheet. Thereafter, the sheet is ejected onto a printed-sheet tray.
Such an image forming apparatus is also structured such that a sheet slacks while the sheet is present between a nip portion of the photosensitive drum and the transfer roller (a transfer position) and the fixing device, for the following reason. When the sheet feeding speed of the fixing device is higher than the sheet feeding speed of the photosensitive drum and the transfer roller, it is impossible that a sheet slacks between the transfer position and the fixing device. In this case, tension is applied to a sheet, and the photosensitive drum and the transfer roller slip. Thereby, the quality of the toner image transferred onto the sheet is lowered. In order to prevent this problem, the sheet feeding speed of the photosensitive drum and the transfer roller is set higher than the sheet feeding speed of the fixing device. Thereby, a sheet slacks between the transfer position and the fixing device, and the sheet is prevented from being subjected to tension.
However, in such an image forming apparatus, if a sheet slacks excessively, the quality of the toner image transferred onto the sheet is lowered. More specifically, if a sheet slacks excessively, the sheet will be returning straight. Accordingly, the part of the sheet present in the fixing device, which is located more downstream in a sheet path, is forced forward and downstream, and the part of the sheet present between the photosensitive drum and the transfer roller, which are located upstream in the sheet path, is forced backward and upstream. Therefore, the photosensitive drum and the transfer roller slip, and consequently, the quality of the toner image transferred onto the sheet is lowered. Therefore, it is desired that a sheet slacks moderately between the transfer position and the fixing device.
In a conventional image forming apparatus, degradation of a toner image is prevented by use of a detection mechanism as will be described below. FIG. 11 shows a photosensitive drum 502, a transfer roller 504 and a fixing device 506 in a conventional image forming apparatus 500.
The image forming apparatus 500 has a detection mechanism 508 as well as the photosensitive drum 502, the transfer roller 504 and the fixing device 506. The photosensitive drum 502, the transfer roller 504 and the fixing device 506 operate as described above, and descriptions thereof are omitted here. The detection mechanism 508 is to detect a sheet P slacking, and the detection mechanism 508 comprises a lever 508a and a sensor 508b. The lever 508a is a stick-like member. The center of the lever 508a in the lengthwise direction is fixed, and the lever 508a is capable of pivoting on a body of the image forming apparatus 500 in a direction shown by the arrow. The sensor 508b is disposed near the lever 508a, and the sensor 508b comprises a light emitting element and a light receiving element. When the sheet P is tense, the lever 508a interrupts the light emitting element and the light receiving element of the sensor 508b. When the sheet P slacks, the lever 508a is pushed by the sheet P and pivots clockwise. Thereby, as shown in FIG. 11, the lever 508a retreats from the interrupting position between the light emitting element and the light receiving element. In this way, in the image forming apparatus 500, the detection mechanism 508 detects the sheet P slack, and a control section (not shown) controls the sheet feeding speed of the fixing device 506 in accordance with the output from the detection mechanism 508 so that the sheet P will slack moderately.
In the image forming apparatus 500, however, it is difficult to certainly prevent degradation of a toner image for the following reason. Various kinds of sheets can be used as the sheet P. Specifically, various materials of sheets, for example, cardboard sheets, ordinary sheets, etc. and various sizes of sheets, for example, A4-sized sheets, A3-sized sheets, etc. can be used as the sheet P. The ease of slacking of the sheet P depends on the kind of the sheet P, and even when the sheet P slacks in a degree, it will or will not result in degradation of a toner image on the sheet P.
In this apparatus 500, the detection mechanism 508 can detect the sheet P slack only in a certain degree. Therefore, when the sheet P is a type of sheet that is easy to slack, detection of a slack of the sheet P serves the purpose of preventing degradation of a toner image. On the other hand, when the sheet P is a type of sheet that is hard to slack, although the sheet P is in such a condition to cause degradation of a toner image, the detection mechanism may not detect the condition, resulting in a failure in preventing degradation of a toner image.
In a conventional image forming apparatus disclosed by Japanese Patent Laid-Open Publication No. 8-220928, the conditions of the heat/pressure treatment carried out by the fixing device are changed depending on the kind of the recording paper. However, Japanese Patent Laid-Open Publication No. 8-220928 is silent about preventing degradation of a toner image by making a slack of the sheet P.